1. Field of the Invention
This invention relates to a tilt adjustment mechanism for adjusting the tilt of an object lens driving device driving the object lens in the focusing or tracking direction, in an optical pick up device employed for recording, reproducing and/of erasure of a magneto-optical disk.
2. Description of Background Art
Referring to FIG. 1, an example of a magneto-optical disk as the background of the present invention will be. explained. FIG. 1 is a diagramatic side elevation showing an example of a conventional magneto-optical disk device forming the background of the present invention.
Referring to FIG. 1, the magneto-optical disk 1 is driven into rotating by an electric motor 2. A laser light 7 emitted from a laser light source 3a is reflected by a mirror 3b. The laser light 7 is passed through an object lens 6 so as to be converged on the surface of a disk-shaped recording medium enclosed in the magneto-optical disk 1. The recording, reproduction and/or erasure of the information is performed optically by the medium of light spots formed in this manner on the disk-shaped recording medium 1a. An object lens driving device 4 drives the object lens 6 perpendicularly and horizontally for performing follow-up control of the converging position of the laser light 7, that is, the position of formation of light spots, on the recording track of the disk shaped recording medium 1a. A housing 3 is provided with an optical system comprised of the laser light source 3a, the mirror 3b or the like. The optical pick up device, that is, the optical head, is made up of the housing 3 and the object lens driving device 4. A coil of an electromagnet 5 forms a magnetic field for recording or erasure of the information on the disk shaped recording medium 1a.
It is now assumed that, in the above described magneto-optical disk device, the central axis of the object lens 6 has a tilt with respect to an optical axis 7a of the laser light emitted from the laser light source 3a and reflected by the mirror 3b. In this case, the laser light 7 proceeding from the object lens 6 to the magneto-optical disk 1 undergoes an aberration which means the deviation of the wave front of the light with respect to a predetermined wave front. If the aberration occurs, the beam throttled by the object lens 6 can not be throttled to a small size, or a true-circular light spot can not be formed on the disk shaped recording medium 1a. In this case, the laser light 7 can not be converged sufficiently, thus causing cross-talk among the information data recorded on adjacent recording tracks on the disk shaped recording medium 1a. The cross-talk means the light of the light spot formed on the disk shaped recording medium 1a reaching the adjacent track by the aberration in case the light spot is not truly circular so that the signal on the adjacent track is read.
Conventionally, for overcoming the above inconvenience, a spacer 8 is interposed between the object lens driving device 4 and the housing 3, as shown in FIG. 1. The tilt of the object lens driving device 4 with respect to the housing 3 is adjusted by adjusting the thickness of the interposed spacer 8. In this manner, the central axis of the object lens 6 is adjusted so as to be substantially parallel to the optical axis 7a.
FIG. 2A is a plan view showing the tilt adjustment mechanism of the conventional object lens driving device, and FIG. 2B is a diagramatic side elevation showing the tilt adjustment mechanism. In this tilt adjustment mechanism, a spherical concave portion 9a is formed on the upper surface of the housing 3. A mating spherical projection 9b is formed on the bottom surface of the object lens driving device 4. A coil spring 11 is interposed between the object lens driving device 4 and screws 10a, 10b. The object lens driving device 4 is supported by adjustment screws 10c, 10d on the housing 3. The tilt of the object lens driving device 4 with respect to the housing 3 may be adjusted by turning the screws 10c, 10d. After such adjustment, the object lens driving device 4 is secured by tightening the screws 10a, 10b or injecting an adhesive into the spherical concave portion 9a.
However, such tilt adjustment by the spacer 8 necessitates a difficult adjustment operation and much adjustment time resulting in increased production costs. Also, in such adjustment by the spherical concave portion 9a and the mating spherical projection 9b, it is necessary to provide a region for formation of the spherical concave portion 9a in the housing 3. This results in the increased overall height of the device thus obstructing the reduction in size of the overall device.
On the other hand, the Japanese Patent Laying-Open Gazette No. 144423/1988 discloses another tilt adjustment mechanism. FIG. 3A is a cross sectional front view showing this tilt adjustment mechanism, and FIG. 3B is a partial top plan view of the tilt adjustment mechanism shown in FIG. 3A. Referring to these figures, a convex shaped supporting shaft 9d is provided on the lower surface of the object lens driving device 4. A mating concave shaped bearing 9c is provided on the upper surface of the housing 3. The object lens driving device 4 is provided with two screw hole sections 4a disposed on the lower side of the device 4, and two screw hole sections 4b provided on the upper side sections of the device 4. The driving device 4 is secured to the housing by an adjustment screw inserted into the screw hole section 4a by the medium of a compression spring 11a and another adjustment screw 10b inserted into the screw hole section 4b by the medium of another compression spring 11b. These adjustment screws 10a and 10b may be tightened or loosened with the contact point between the supporting shaft 9d and the bearing 9c as the fulcrum or supporting point for adjusting the tilt of the object lens driving device with respect to the housing 3 in the directions shown by the arrow marks A and B.
However, this tilt adjustment mechanism necessitates two additional components, namely, the supporting shaft and the bearing, resulting in increased assembling time and increased production costs. It is also difficult to perform precision machining after a predetermined length of the foremost part of the supporting shaft is assured with a predetermined arched or V shape. It is also difficult to perform a precise machining of the concave portion of the bearing to a predetermined arched or V shape. Thus, it becomes difficult to control the height position of the contact portions between the supporting shaft and the bearing among the various different devices within a small range of dispersion and, consequently, to control the reference height of the object lens within the range of small dispersion in adjusting the tilt of the object lens driving device with respect to the housing. In addition, the tilt adjustment mechanism is constituted by four adjustment points and one supporting point, resulting in increased assembling and adjustment time and increase production costs.
The Japanese Patent Laying-Open Gazette No. 149839/1988 discloses still another tilt adjustment mechanism. FIG. 4A is a plan view showing the arrangement of the adjustment point and the support point in this tilt adjustment mechanism, and FIG. 4B is a partial sectional view at one adjustment point. FIG. 5A is a plan view showing the arrangement of an adjustment point and a support point for another example of the tilt adjustment mechanism disclosed in this Laying-Open publication, and FIG. 5B is a partial sectional view at one adjustment point shown in FIG. 5A.
Referring to FIGS. 4A and 4B, a base 13 supporting the object lens driving device has its tilt adjusted with respect to the housing 3 about one supporting point 9e as center. The base 13 is provided with two adjustment points each formed by adjustment screws 10a, 10b and set screws 10c, 10d. The adjustment screws 10a and 10b may be turned clockwise or counterclockwise to adjust the tilt in the mutually perpendicular directions shown by the arrow marks A and B. For example, referring to FIG. 4B, the tilt is adjusted by turning the adjustment screw 10a, after which the set screw 10c is inserted and tightened to secure the base to the housing 3 with a predetermined distance in between.
Referring to FIGS. 5A and 5B, hollow adjustment screws 10a and 10b may be turned clockwise or counterclockwise to adjust the tilt of the base 13 with respect to the housing 3 in each of the directions shown by the arrow marks A and B. The set screws 10c and 10d are then inserted and tightened to secure the base 13 to the housing 3. At the support point 9e, the base 13 and the housing 3 is provided with a spherical projection and a mating spherical concave portion, respectively.
In the tilt adjustment mechanism shown in FIGS. 4A, 4B, 5A and 5B, it may occur that, when the set screws 10d at one adjustment point is tightened strongly, it is not possible to adjust the tilt by turning adjustment screw 10a at the other adjustment point. Referring to FIG. 4A and 4B, when the set screw 10d at one adjustment point is tightened strongly and the adjustment screw 10a at the other adjustment point is turned counterclockwise from above, that is, the adjustment screw 10a is turned in a direction of reducing the distance between the base 13 and the housing 3, there is formed a gap between the lower surface of the adjustment screw 10a and the upper surface of the housing 3. It is because the set screw 10d at the one adjustment point is tightened strongly. In this manner, there may be occasions wherein the tilt in one direction can not be adjusted by simply turning the adjustment screw 10a at the one adjustment point. In this case, it is not possible to adjust the tilt in the direction of reducing distance between the base 13 and the housing 3 by turning the adjustment screw 10a counterclockwise from the above, unless the two set screws 10c and 10d at the one and the other adjustment points are loosened. Also, as shown in FIG. 5A and 5B, when the set screw 10d at one adjustment point is tightened strongly and the hollow adjustment screw 10a at the other adjustment point is turned clockwise from above, there is formed a gap between the upper surface of the adjustment screw 10a and the lower surface of the base 13. Hence, the problem arises which is similar to that described hereinbefore.
In addition, in the tilt adjustment mechanism shown in FIGS. 5A and 5B, hollow screws are employed as the adjustment screws, resulting in increased production costs. Moreover, the above two tilt adjustment mechanisms make use of mutually registering spherical projections or recesses formed in each of the base 13 and the housing 3 as the supporting point. However, it is difficult to machine the mutually registering spherical projections and concave portions within the range of small dispersion so as to have substantially equal radius of curvature. As a result, in adjusting the tilt of the object lens driving device, it is difficult to control the reference height of the object lenses among the different various devices within the range of small dispersion.